In recent years, energy use has become more and more efficient, and demands are increasingly being made, mainly from transformer manufacturers and the like, for an electrical steel sheet with high flux density and low iron loss.
Flux density can be improved by according crystal orientations of the electrical steel sheet with the Goss orientation.
With regards to iron loss reduction, measures have been devised from the perspectives of increasing purity of the material, high orientation, reduced sheet thickness, addition of Si and Al, magnetic domain refining, and the like. However, iron loss properties generally tend to worsen as the flux density is higher. The reason is that when the crystal orientations are aligned, the magnetostatic energy decreases and, therefore, the magnetic domain width widens, causing eddy current loss to rise.
One solution to this problem is to reduce eddy current loss. Specific methods include a method of applying magnetic domain refining by applying thermal strain to the surface of a steel sheet with a method such as a laser or electron beam. Both methods are known to exhibit an extremely high improving effect in iron loss by irradiation.
For example, JPH7-65106B2 discloses a method of manufacturing an electrical steel sheet having iron loss W17/50 of below 0.8 W/kg due to electron beam irradiation.
Furthermore, JPH3-13293B2 discloses a method of reducing iron loss by applying laser irradiation to an electrical steel sheet.
However, although grain-oriented electrical steel sheets subjected to magnetic domain refining by irradiating laser or electron beam may exhibit good characteristics as a material, there are cases where good characteristics cannot be obtained when manufacturing a transformer using the same grain-oriented electrical steel sheet. Specifically, the problem is that the noise of the transformer increases. In other words, even if iron loss, flux density, magnetostriction and the like measured in the state of a single sheet material are the same, some conditions produce transformers with a loud noise and some conditions produce transformers with a small noise, depending on the pattern of applying thermal strain.
It could therefore be helpful to provide a grain-oriented electrical steel sheet capable of effectively reducing the noise caused by the grain-oriented electrical steel sheet when worked into a transformer.